The present invention is related to an improved high efficiency heat exchanger of the type disclosed in U.S. Pat. Nos. 4,311,191 and 4,311,192, each issued on Jan. 19, 1982 in the name of Gerry Vandervaart and U.S. Pat. No. 4,461,345 issued on July 24, 1984, also in the name of Gerry Vandervaart. The contents of these three patents are incorporated herein by reference, particularly with respect to presently conventional structural and functional characteristics of such prior art heat exchangers.
U.S. Pat. Nos. 4,311,191 and 4,311,192 each disclose a heat exchanger which includes conventional components such as a compressor, indoor and outdoor coils, blowers associated with the coils, a reversing/expansion valve, and appropriate tubing or conduits such that the heat-exchange medium/refrigerant (Freon) can flow in opposite directions through associated conduits during air conditioning/cooling mode on the one hand and heating/heat-augmenting modes on the other. Traditionally, heat exchangers of the type disclosed in these patents only included reversible operation for cooling and heating modes, but in these patents in a heat-augmenting mode a gas burner directs flames against the outdoor A-coil as liquid refrigerant is introduced into the bottom thereof. The liquid refrigerant (Freon) absorbs the heat/Btu's which increases its temperature resulting in a vapor phase exiting the outdoor A-coil at its top which is subsequently transferred to the indoor coil and utilized with its associated blower to heat the interior of the building.
These conventional heat exchangers are extremely efficient up to approximately 5 tons, and this efficiency is attributed primarily to the fact that the outdoor A-coil is relatively short in height (20 inches high), the heat of the flame is generally intense and is "trapped" within the confines of the A-coil, and because the liquid refrigerant is introduced into the bottom of the A-coil which immediately absorbs a relatively great proportion of the BTU's at the lower end of the A-coil than at the upper end thereof which creates equalization of coil pressure/temperature and attendant liquid refrigerant to boiled-off vapor transfer.
Obviously, if one were to desire a higher capacity heat exchanger, one would expect that all need be done would be to increase the capacity of the outdoor A-coil by, for example, merely increasing its height (or its length) with other components being proportionately sized. However, there was no proportionate increase in efficiency found in actual practice when the conventional 20 inch high outdoor A-coil was replaced by a 36 inch high coil. Instead the efficiency of the heat pump in all modes of operation, but particularly the heat and heat-augmented modes of operation, was reduced. So long as the outdoor A-coil was relatively small and the flame was intense and generally trapped within the A-coil, except for its free flow through the coils of the legs thereof, the liquid refrigerant boiled-off generally uniformly, but as the temperature drops the refrigerant does not boil-off at the same rate of speed as the flow of the refrigerant through the tubing. Consequently, in the smaller sized outdoor A-coil of the patented system, the refrigerant at the bottom of the outdoor A-coil boils off from its liquid to its vapor state as it moves upwardly with relative uniformity and ease. However, in the larger outdoor A-coil there is insufficient liquid refrigerant to maximize boil-off. While an appropriate expansion device could be used to fill the tubing to such a point where it flows out the back into the compressor in a conventional manner, this failed to maintain necessary generally constant pressure/temperature throughout the outdoor A-coil, and particularly the two "legs" or sides thereof. The ability to maintain such pressure/temperature balance substantially decreased in the large (36 inch high) outdoor A-coil. The liquid refrigerant in the larger outdoor A-coil tended more so to fill the colder side or leg of the outdoor A-coil (because of a lesser amount of air flow therethrough during the heat pump cycle), and as the burner flame came on, the easier path of travel for the heat is the side of the outdoor A-coil with the least amount of refrigerant therein. Thus, this automatically created an imbalance which likewise destroyed the heat transfer efficiency between the relatively intense gas flame and the liquid refrigerant. Quite simply, while in the smaller outdoor A-coil's heating and heat-augmented modes, the temperature and, therefore, the pressure of the refrigerant could be balanced throughout the outdoor A-coil it was relatively impossible to boil-off the liquid or refrigerant into its low pressure vapor state in both legs of the larger/higher outdoor a-coil.